Holographic waveguides incorporating birefringence control and methods for their fabrication

ABSTRACT

Many embodiments in accordance with the invention are directed towards waveguides implementing birefringence control. In some embodiments, the waveguide includes a birefringent grating layer and a birefringence control layer. In further embodiments, the birefringence control layer is compact and efficient. Such structures can be utilized for various applications, including but not limited to: compensating for polarization related losses in holographic waveguides; providing three-dimensional LC director alignment in waveguides based on Bragg gratings; and spatially varying angular/spectral bandwidth for homogenizing the output from a waveguide. In some embodiments, a polarization-maintaining, wide-angle, and high-reflection waveguide cladding with polarization compensation is implemented for grating birefringence. In several embodiments, a thin polarization control layer is implemented for providing either quarter wave or half wave retardation.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application is a continuation of U.S. patent applicationSer. No. 16/906,872 entitled “Holographic Waveguides IncorporatingBirefringence Control and Methods for Their Fabrication,” filed Jun. 19,2020, which is a continuation of U.S. patent application Ser. No.16/357,233 entitled “Holographic Waveguides Incorporating BirefringenceControl and Methods for Their Fabrication,” filed Mar. 18, 2019, whichclaims the benefit of and priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application No. 62/643,977 entitled “HolographicWaveguides Incorporating Birefringence Control and Methods for TheirFabrication,” filed Mar. 16, 2018. The disclosures which are herebyincorporated by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present disclosure relates to optical waveguides and moreparticularly to waveguide displays using birefringent gratings.

BACKGROUND OF THE INVENTION

Waveguides can be referred to as structures with the capability ofconfining and guiding waves (i.e., restricting the spatial region inwhich waves can propagate). One subclass includes optical waveguides,which are structures that can guide electromagnetic waves, typicallythose in the visible spectrum. Waveguide structures can be designed tocontrol the propagation path of waves using a number of differentmechanisms. For example, planar waveguides can be designed to utilizediffraction gratings to diffract and couple incident light into thewaveguide structure such that the in-coupled light can proceed to travelwithin the planar structure via total internal reflection (“TIR”).

Fabrication of waveguides can include the use of material systems thatallow for the recording of holographic optical elements within thewaveguides. One class of such material includes polymer dispersed liquidcrystal (“PDLC”) mixtures, which are mixtures containingphotopolymerizable monomers and liquid crystals. A further subclass ofsuch mixtures includes holographic polymer dispersed liquid crystal(“HPDLC”) mixtures. Holographic optical elements, such as volume phasegratings, can be recorded in such a liquid mixture by illuminating thematerial with two mutually coherent laser beams. During the recordingprocess, the monomers polymerize and the mixture undergoes aphotopolymerization-induced phase separation, creating regions denselypopulated by liquid crystal micro-droplets, interspersed with regions ofclear polymer. The alternating liquid crystal-rich and liquidcrystal-depleted regions form the fringe planes of the grating.

Waveguide optics, such as those described above, can be considered for arange of display and sensor applications. In many applications,waveguides containing one or more grating layers encoding multipleoptical functions can be realized using various waveguide architecturesand material systems, enabling new innovations in near-eye displays foraugmented reality (“AR”) and virtual reality (“VR”), compact heads-updisplays (“HUDs”) for aviation and road transport, and sensors forbiometric and laser radar (“LIDAR”) applications.

SUMMARY OF THE INVENTION

Following below are more detailed descriptions of various conceptsrelated to, and embodiments of, an inventive optical display and methodsfor displaying information. It should be appreciated that variousconcepts introduced above and discussed in greater detail below may beimplemented in any of numerous ways, as the disclosed concepts are notlimited to any particular manner of implementation. Examples of specificimplementations and applications are provided primarily for illustrativepurposes. A more complete understanding of the invention can be obtainedby considering the following detailed description in conjunction withthe accompanying drawings, wherein like index numerals indicate likeparts. For purposes of clarity, details relating to technical materialthat is known in the technical fields related to the invention have notbeen described in detail.

One embodiment includes a waveguide including at least one waveguidesubstrate, at least one birefringent grating; at least one birefringencecontrol layer, a light source for outputting light, an input coupler fordirecting the light into total internal reflection paths within thewaveguide, and an output coupler for extracting light from thewaveguide, wherein the interaction of the light with the birefringencecontrol layer and the birefringent grating provides a predefinedcharacteristic of light extracted from the waveguide.

In another embodiment, the interaction of light with the birefringencecontrol layer provides at least one of: an angular or spectral bandwidthvariation, a polarization rotation, a birefringence variation, anangular or spectral dependence of at least one of beam transmission orpolarization rotation, and a light transmission variation in at leastone direction in the plane of the waveguide substrate.

In a further embodiment, the predefined characteristic varies across thewaveguide.

In still another embodiment, the predefined characteristic results fromthe cumulative effect of the interaction of the light with thebirefringence control layer and the birefringent grating along at leastone direction of light propagation within the waveguide.

In a still further embodiment, the predefined characteristic includes atleast one of: uniform illumination and uniform polarization over theangular range of the light.

In yet another embodiment, the birefringence control layer providescompensation for polarization rotation introduced by the birefringentgrating along at least one direction of light propagation within thewaveguide.

In a yet further embodiment, the birefringence control layer is a liquidcrystal and polymer material system.

In another additional embodiment, the birefringence control layer is aliquid crystal and polymer system aligned using directional ultravioletradiation.

In a further additional embodiment, the birefringence control layer isaligned by at least one of: electromagnetic radiation, electrical ormagnetic fields, mechanical forces, chemical reaction, and thermalexposure.

In another embodiment again, the birefringence control layer influencesthe alignment of LC directors in a birefringent grating formed in aliquid crystal and polymer system.

In a further embodiment again, the birefringence control layer has ananisotropic refractive index.

In still yet another embodiment, the birefringence control layer isformed on at least one internal or external optical surface of thewaveguide.

In a still yet further embodiment, the birefringence control layerincludes at least one stack of refractive index layers disposed on atleast one optical surface of the waveguide, wherein at least one layerin the stack of refractive index layers has an isotropic refractiveindex and at least one layer in the stack of refractive index layers hasan anisotropic refractive index.

In still another additional embodiment, the birefringence control layerprovides a high reflection layer.

In a still further additional embodiment, the birefringence controllayer provides optical power.

In still another embodiment again, the birefringence control layerprovides an environmental isolation layer for the waveguide.

In a still further embodiment again, the birefringence control layer hasa gradient index structure.

In yet another additional embodiment, the birefringence control layer isformed by stretching a layer of an optical material to spatially varyits refractive index in the plane of the waveguide substrate.

In a yet further additional embodiment, the light source providescollimated light in angular space.

In yet another embodiment again, at least one of the input coupler andoutput coupler includes a birefringent grating.

In a yet further embodiment again, the birefringent grating is recordedin a material system including at least one polymer and at least oneliquid crystal.

In another additional embodiment again, the at least one birefringentgrating includes at least one birefringent grating for providing atleast one of the functions of: beam expansion in a first direction, beamexpansion in a second direction and light extraction from the waveguide,and coupling light from the source into a total internal reflection pathin the waveguide.

In a further additional embodiment again, the light source includes alaser, and the alignment of LC directors in the birefringent gratingspatially vary to compensate for illumination banding.

A still yet another additional embodiment includes a method offabricating a waveguide, the method including providing a firsttransparent substrate, depositing a layer of grating recording material,exposing the layer of grating recording material to form a gratinglayer, forming a birefringence control layer, and applying a secondtransparent substrate.

In a still yet further additional embodiment, the layer of gratingrecording material is deposited onto the substrate, the birefringencecontrol layer is formed on the grating layer, and the second transparentsubstrate is applied over the birefringence control layer.

In yet another additional embodiment again, the layer of gratingrecording material is deposited onto the substrate, the secondtransparent substrate is applied over the grating layer, and thebirefringence control layer is formed on second transparent substrate.

In a yet further additional embodiment again, the birefringence controllayer is formed on the first transparent substrate, the layer of gratingrecording material is deposited onto the birefringence control layer,and the second transparent substrate is applied over the grating layer.

In still yet another embodiment again, the method further includesdepositing a layer of liquid crystal polymer material and aligning theliquid crystal polymer material using directional UV light, wherein thelayer of grating recording material is deposited onto the substrate andthe second transparent substrate is applied over the aligned liquidcrystal polymer layer.

In a still yet further embodiment again, the layer of liquid crystalpolymer material is deposited onto one of either the grating layer orthe second transparent substrate.

In still yet another additional embodiment again, the layer of liquidcrystal polymer material is deposited onto the first transparentsubstrate, the layer of grating recording material is deposited onto thealigned liquid crystal polymer material, and the second transparentsubstrate is applied over the grating layer.

Additional embodiments and features are set forth in part in thedescription that follows, and in part will become apparent to thoseskilled in the art upon examination of the specification or may belearned by the practice of the invention. A further understanding of thenature and advantages of the present invention may be realized byreference to the remaining portions of the specification and thedrawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bebetter understood by reference to the following detailed descriptionwhen considered in conjunction with the accompanying data and figures,wherein:

FIG. 1 conceptually illustrates a schematic cross section view of awaveguide incorporating a birefringent grating and birefringence controllayer in accordance with an embodiment of the invention.

FIG. 2 conceptually illustrates a schematic cross section view of awaveguide incorporating a birefringent grating and birefringence controllayer for compensating grating birefringence in accordance with someembodiments of the invention.

FIG. 3 conceptually illustrates a schematic cross section view of awaveguide incorporating a birefringent grating and birefringence controllayer for providing uniform output illumination from the waveguide inaccordance with an embodiment of the invention.

FIG. 4 conceptually illustrates a schematic cross section view of abirefringence control layer formed by a multilayer structure combiningisotropic and anisotropic index layers in accordance with an embodimentof the invention.

FIG. 5 conceptually illustrates a schematic cross section view of abirefringence control layer formed by a multilayer structure combiningisotropic and anisotropic index layers integrated with a birefringentgrating layer in accordance with an embodiment of the invention.

FIG. 6 conceptually illustrates a plan view of a dual expansionwaveguide with birefringent control layers in accordance with anembodiment of the invention.

FIG. 7 conceptually illustrates a schematic cross section view of awaveguide incorporating a birefringent grating and birefringence controllayer for correcting birefringence introduced by an optical element inthe output light path from the waveguide in accordance with anembodiment of the invention.

FIG. 8 conceptually illustrates a schematic plan view of an apparatusfor aligning a birefringence control layer by applying forces to theedges of the layer in accordance with an embodiment of the invention.

FIGS. 9A-9F conceptually illustrate the process steps and apparatus forfabricating a waveguide containing a birefringent grating and abirefringence control layer in accordance with various embodiments ofthe invention.

FIGS. 10A-10F conceptually illustrate the process steps and apparatusfor fabricating a waveguide containing a birefringent grating with abirefringence control layer applied to an outer surface of the waveguidein accordance with various embodiments of the invention.

FIGS. 11A-11F conceptually illustrate the process steps and apparatusfor fabricating a waveguide containing a birefringent grating and abirefringence control layer in accordance with various embodiments ofthe invention.

FIG. 12 conceptually illustrates a flow chart showing a method offabricating a waveguide containing a birefringent grating and abirefringence control layer in accordance with an embodiment of theinvention.

FIG. 13 conceptually illustrates a flow chart showing a method offabricating a waveguide containing a birefringent grating and abirefringence control layer applied to an outer surface of the waveguidein accordance with an embodiment of the invention.

FIG. 14 conceptually illustrates a flow chart showing a method offabricating a waveguide containing a birefringent grating and abirefringence control layer where forming the birefringence controllayer is carried out before the recording of the grating layer inaccordance with an embodiment of the invention.

FIG. 15 conceptually illustrates a schematic side view of a waveguidewith a birefringence control layer applied at the waveguide to airinterface in accordance with an embodiment of the invention.

FIG. 16 conceptually illustrates a schematic side view of a waveguidewith a birefringence control layer that isolates the waveguide from itsenvironment applied to the waveguide to air interface in accordance withan embodiment of the invention.

FIG. 17 conceptually illustrates a schematic side view of an apparatusfor fabricating a structure containing a birefringent grating layeroverlaying a birefringence control layer where the grating recordingbeams propagate through the birefringence control layer in accordancewith an embodiment of the invention.

FIG. 18 conceptually illustrates a schematic side view of an apparatusfor fabricating a structure containing a birefringence control layeroverlaying a birefringent grating layer where the birefringence controllayer is aligned by UV radiation propagating through the grating inaccordance with an embodiment of the invention.

FIG. 19 conceptually illustrates a cross section of waveguide containingsubstrates sandwiching a grating layer.

FIG. 20 conceptually illustrates a waveguide with a quarter wavepolarization layer inserted in accordance with an embodiment of theinvention.

FIG. 21 conceptually illustrates a schematic cross section view showinga portion of a waveguide illustrating the use of a quarter wavepolarization layer with a RKV grating in accordance with an embodimentof the invention.

FIG. 22 conceptually illustrates a polarization layer architecturecontaining an LCP quarter wave cell and a reactive monomer liquidcrystal mixture (RMLCM) cell separated by index matching oil layer inaccordance with an embodiment of the invention.

FIG. 23 conceptually illustrates an example of a polarizationarchitecture based on a grating cell with the RMLCM grating materiallayer in direct contact with a bare LCP film in accordance with anembodiment of the invention.

FIG. 24 conceptually illustrates a cross section view schematicallyshowing an example of polarization layer architecture in which a bareLCP layer is bonded to a bare RMLCM layer in accordance with anembodiment of the invention.

FIG. 25 conceptually illustrates a cross section view schematicallyshowing an example of a polarization layer architecture using a RMLCMlayer as a polarization layer in accordance with an embodiment of theinvention.

FIG. 26 conceptually illustrates an example of a polarization layerarchitecture that includes a feature for compensating for polarizationrotation introduced by birefringent gratings in accordance with anembodiment of the invention.

FIG. 27 conceptually illustrates a plan view schematically showing awaveguide display incorporating the features of the embodiment of FIG.26 in accordance with an embodiment of the invention.

FIGS. 28 and 29 conceptually illustrate cross section viewsschematically showing examples of polarization layer architecturescontaining an upper substrate, an LCP layer with hard encapsulationlayer, a RMLCM layer, and a lower substrate in accordance with variousembodiments of the invention.

FIG. 30 conceptually illustrates a plan view schematically showing afirst example of a two-region polymer film in accordance with anembodiment of the invention.

FIG. 31 conceptually illustrates a plan view schematically showing asecond example of a two-region polymer film in accordance with anembodiment of the invention.

FIG. 32 conceptually illustrates a plan view schematically showing athird example of a two-region polymer film in accordance with anembodiment of the invention.

FIG. 33 conceptually illustrates a drawing showing a clear aperturelayout in accordance with an embodiment of the invention.

FIG. 34 conceptually illustrates a plan view schematically showing awaveguide containing input, fold, and output gratings including theK-vectors and alignment layer fast axis directions for each grating inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of describing embodiments, some well-known features ofoptical technology known to those skilled in the art of optical designand visual displays have been omitted or simplified in order to notobscure the basic principles of the invention. Unless otherwise stated,the term “on-axis” in relation to a ray or a beam direction refers topropagation parallel to an axis normal to the surfaces of the opticalcomponents described in relation to the invention. In the followingdescription the terms light, ray, beam, and direction may be usedinterchangeably and in association with each other to indicate thedirection of propagation of electromagnetic radiation along rectilineartrajectories. The term light and illumination may be used in relation tothe visible and infrared bands of the electromagnetic spectrum. Parts ofthe following description will be presented using terminology commonlyemployed by those skilled in the art of optical design. In the followingdescription, the term grating may be used to refer to any kind ofdiffractive structure used in a waveguide, including holograms and Braggor volume holograms. The term grating may also encompass a grating thatincludes of a set of gratings. For example, in some embodiments theinput grating and output grating each include two or more gratingsmultiplexed into a single layer. For illustrative purposes, it is to beunderstood that the drawings are not drawn to scale unless statedotherwise.

Referring generally to the drawings, systems and methods relating towaveguide applications incorporating birefringence control in accordancewith various embodiments of the invention are illustrated. Birefringenceis the optical property of a material having a refractive index thatdepends on the polarization and propagation direction of light. Abirefringent grating can be referred to as a grating having suchproperties. In many cases, the birefringent grating is formed in aliquid crystal polymer material system such as but not limited to HPDLCmixtures. The polarization properties of such a grating can depend onaverage relative permittivity and relative permittivity modulationtensors.

Many embodiments in accordance with the invention are directed towardswaveguides implementing birefringence control. In some embodiments, thewaveguide includes a birefringent grating layer and a birefringencecontrol layer. In further embodiments, the birefringence control layeris compact and efficient. Such structures can be utilized for variousapplications, including but not limited to: compensating forpolarization related losses in holographic waveguides; providingthree-dimensional LC director alignment in waveguides based on Bragggratings; and spatially varying angular/spectral bandwidth forhomogenizing the output from a waveguide. In some embodiments, apolarization-maintaining, wide-angle, and high-reflection waveguidecladding with polarization compensation is implemented for gratingbirefringence. In several embodiments, a thin polarization control layeris implemented for providing either quarter wave or half waveretardation. In a number of embodiments, a polarization-maintaining,wide-angle birefringence control layer is implemented for modifying thepolarization output of a waveguide to balance the birefringence of anexternal optical element used with the waveguide.

In many embodiments, the waveguide includes at least one input gratingand at least one output grating. In further embodiments, the waveguidecan include additional gratings for various purposes, such as but notlimited to fold gratings for beam expansion. The input grating andoutput grating may each include multiplexed gratings. In someembodiments, the input grating and output grating may each include twooverlapping gratings layers that are in contact or vertically separatedby one or more thin optical substrate. In some embodiments, the gratinglayers are sandwiched between glass or plastic substrates. In someembodiments two or more such gratings layers may form a stack withinwhich total internal reflection occurs at the outer substrate and airinterfaces. In some embodiments, the waveguide may include just onegrating layer. In some embodiments, electrodes may be applied to facesof the substrates to switch gratings between diffracting and clearstates. The stack may further include additional layers such as beamsplitting coatings and environmental protection layers. The input andoutput gratings shown in the drawings may be provided by any of theabove described grating configurations. Advantageously, the input andoutput gratings can be designed to have common surface grating pitch. Incases where the waveguide contains grating(s) in addition to the inputand output gratings, the gratings can be designed to have gratingpitches such that the vector sum of the grating vectors is substantiallyzero. The input grating can combine gratings orientated such that eachgrating diffracts a polarization of the incident unpolarized light intoa waveguide path. The output gratings can be configured in a similarfashion such that the light from the waveguide paths is combined andcoupled out of the waveguide as unpolarized light. Each grating ischaracterized by at least one grating vector (or K-vector) in 3D space,which in the case of a Bragg grating is defined as the vector normal tothe Bragg fringes. The grating vector can determine the opticalefficiency for a given range of input and diffracted angles. In someembodiments, the waveguide includes at least one surface relief grating.Waveguide gratings structures, materials systems, and birefringencecontrol are discussed below in further detail.

Switchable Bragg Gratings

Optical structures recorded in waveguides can include many differenttypes of optical elements, such as but not limited to diffractiongratings. In many embodiments, the grating implemented is a Bragggrating (also referred to as a volume grating). Bragg gratings can havehigh efficiency with little light being diffracted into higher orders.The relative amount of light in the diffracted and zero order can bevaried by controlling the refractive index modulation of the grating, aproperty that is can be used to make lossy waveguide gratings forextracting light over a large pupil. One class of gratings used inholographic waveguide devices is the Switchable Bragg Grating (“SBG”).SBGs can be fabricated by first placing a thin film of a mixture ofphotopolymerizable monomers and liquid crystal material between glassplates or substrates. In many cases, the glass plates are in a parallelconfiguration. One or both glass plates can support electrodes,typically transparent tin oxide films, for applying an electric fieldacross the film. The grating structure in an SBG can be recorded in theliquid material (often referred to as the syrup) throughphotopolymerization-induced phase separation using interferentialexposure with a spatially periodic intensity modulation. Factors such asbut not limited to control of the irradiation intensity, componentvolume fractions of the materials in the mixture, and exposuretemperature can determine the resulting grating morphology andperformance. As can readily be appreciated, a wide variety of materialsand mixtures can be used depending on the specific requirements of agiven application. In many embodiments, HPDLC material is used. Duringthe recording process, the monomers polymerize and the mixture undergoesa phase separation. The LC molecules aggregate to form discrete orcoalesced droplets that are periodically distributed in polymer networkson the scale of optical wavelengths. The alternating liquid crystal-richand liquid crystal-depleted regions form the fringe planes of thegrating, which can produce Bragg diffraction with a strong opticalpolarization resulting from the orientation ordering of the LC moleculesin the droplets.

The resulting volume phase grating can exhibit very high diffractionefficiency, which can be controlled by the magnitude of the electricfield applied across the film. When an electric field is applied to thegrating via transparent electrodes, the natural orientation of the LCdroplets can change, causing the refractive index modulation of thefringes to lower and the hologram diffraction efficiency to drop to verylow levels. Typically, the electrodes are configured such that theapplied electric field will be perpendicular to the substrates. In anumber of embodiments, the electrodes are fabricated from indium tinoxide (“ITO”). In the OFF state with no electric field applied, theextraordinary axis of the liquid crystals generally aligns normal to thefringes. The grating thus exhibits high refractive index modulation andhigh diffraction efficiency for P-polarized light. When an electricfield is applied to the HPDLC, the grating switches to the ON statewherein the extraordinary axes of the liquid crystal molecules alignparallel to the applied field and hence perpendicular to the substrate.In the ON state, the grating exhibits lower refractive index modulationand lower diffraction efficiency for both S- and P-polarized light.Thus, the grating region no longer diffracts light. Each grating regioncan be divided into a multiplicity of grating elements such as forexample a pixel matrix according to the function of the HPDLC device.Typically, the electrode on one substrate surface is uniform andcontinuous, while electrodes on the opposing substrate surface arepatterned in accordance to the multiplicity of selectively switchablegrating elements.

One of the known attributes of transmission SBGs is that the LCmolecules tend to align with an average direction normal to the gratingfringe planes (i.e., parallel to the grating or K-vector). The effect ofthe LC molecule alignment is that transmission SBGs efficiently diffractP polarized light (i.e., light with a polarization vector in the planeof incidence), but have nearly zero diffraction efficiency for Spolarized light (i.e., light with the polarization vector normal to theplane of incidence). As a result, transmission SBGs typically cannot beused at near-grazing incidence as the diffraction efficiency of anygrating for P polarization falls to zero when the included angle betweenthe incident and reflected light is small. In addition, illuminationlight with non-matched polarization is not captured efficiently inholographic displays sensitive to one polarization only.

HPDLC Material Systems

HPDLC mixtures in accordance with various embodiments of the inventiongenerally include LC, monomers, photoinitiator dyes, and coinitiators.The mixture (often referred to as syrup) frequently also includes asurfactant. For the purposes of describing the invention, a surfactantis defined as any chemical agent that lowers the surface tension of thetotal liquid mixture. The use of surfactants in HPDLC mixtures is knownand dates back to the earliest investigations of HPDLCs. For example, apaper by R. L Sutherland et al., SPIE Vol. 2689, 158-169, 1996, thedisclosure of which is incorporated herein by reference, describes aPDLC mixture including a monomer, photoinitiator, coinitiator, chainextender, and LCs to which a surfactant can be added. Surfactants arealso mentioned in a paper by Natarajan et al, Journal of NonlinearOptical Physics and Materials, Vol. 5 No. I 89-98, 1996, the disclosureof which is incorporated herein by reference. Furthermore, U.S. Pat. No.7,018,563 by Sutherland; et al., discusses polymer-dispersed liquidcrystal material for forming a polymer-dispersed liquid crystal opticalelement including: at least one acrylic acid monomer; at least one typeof liquid crystal material; a photoinitiator dye; a coinitiator; and asurfactant. The disclosure of U.S. Pat. No. 7,018,563 is herebyincorporated by reference in its entirety.

The patent and scientific literature contains many examples of materialsystems and processes that can be used to fabricate SBGs, includinginvestigations into formulating such material systems for achieving highdiffraction efficiency, fast response time, low drive voltage, and soforth. U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No.5,751,452 by Tanaka et al. both describe monomer and liquid crystalmaterial combinations suitable for fabricating SBG devices. Examples ofrecipes can also be found in papers dating back to the early 1990s. Manyof these materials use acrylate monomers, including:

-   -   R. L. Sutherland et al., Chem. Mater. 5, 1533 (1993), the        disclosure of which is incorporated herein by reference,        describes the use of acrylate polymers and surfactants.        Specifically, the recipe includes a crosslinking multifunctional        acrylate monomer; a chain extender N-vinyl pyrrolidinone, LC E7,        photo-initiator rose Bengal, and coinitiator N-phenyl glycine.        Surfactant octanoic acid was added in certain variants.    -   Fontecchio et al., SID 00 Digest 774-776, 2000, the disclosure        of which is incorporated herein by reference, describes a UV        curable HPDLC for reflective display applications including a        multi-functional acrylate monomer, LC, a photoinitiator, a        coinitiators, and a chain terminator.    -   Y. H. Cho, et al., Polymer International, 48, 1085-1090, 1999,        the disclosure of which is incorporated herein by reference,        discloses HPDLC recipes including acrylates.    -   Karasawa et al., Japanese Journal of Applied Physics, Vol. 36,        6388-6392, 1997, the disclosure of which is incorporated herein        by reference, describes acrylates of various functional orders.    -   T. J. Bunning et al., Polymer Science: Part B: Polymer Physics,        Vol. 35, 2825-2833, 1997, the disclosure of which is        incorporated herein by reference, also describes multifunctional        acrylate monomers.    -   G. S. Iannacchione et al., Europhysics Letters Vol. 36 (6).        425-430, 1996, the disclosure of which is incorporated herein by        reference, describes a PDLC mixture including a penta-acrylate        monomer, LC, chain extender, coinitiators, and photoinitiator.

Acrylates offer the benefits of fast kinetics, good mixing with othermaterials, and compatibility with film forming processes. Sinceacrylates are cross-linked, they tend to be mechanically robust andflexible. For example, urethane acrylates of functionality 2 (di) and 3(tri) have been used extensively for HPDLC technology. Higherfunctionality materials such as penta and hex functional stems have alsobeen used.

Overview of Birefringence

Holographic waveguides based on HPDLC offer the benefits of switchingcapability and high index modulation, but can suffer from the inherentbirefringence resulting from the alignment of liquid crystal directorsalong grating vectors during the LC-polymer phase separation. While thiscan lead to a large degree of polarization selectivity, which can beadvantageous in many applications, adverse effects such as polarizationrotation can occur in gratings designed to fold and expand thewaveguided beam in the plane of the waveguide (known as fold gratings).This polarization rotation can lead to efficiency losses and outputlight nonuniformity.

Two common approaches for modifying the alignment of LC directorsinclude rubbing and the application of an alignment layer. Typically, bysuch means, LC directors in a plane parallel to the alignment layer canbe realigned within the plane. In HPDLC Bragg gratings, the problem ismore challenging owing to the natural alignment of LC directors alonggrating K-vectors, making director alignment in all but the simplestgratings a complex three-dimensional problem and rendering conventionaltechniques using rubbing or polyamide alignment layers impractical.Other approaches can include applying electric fields, magnetic fields,and mechanical pressure during curing. These approaches have been shownto have limited success when applied to reflection gratings. However,such techniques typically do not easily translate to transmission Bragggrating waveguides.

A major design challenge in waveguides is the coupling of image contentfrom an external projector into the waveguide efficiently and in such away that the waveguide image is free from chromatic dispersion andbrightness non-uniformity. To overcome chromatic dispersion and toachieve the respectable collimation, the use of lasers can beimplemented. However, lasers can suffer from the problem of pupilbanding artifacts, which manifest themselves as output illuminationnon-uniformity. Banding artifacts can form when the collimated pupil isreplicated (expanded) in a TIR waveguide. In basic terms, the lightbeams diffracted out of the waveguide each time the beam interacts withthe grating can have gaps or overlaps, leading to an illuminationripple. In many cases, the degree of ripple is a function of fieldangle, waveguide thickness, and aperture thickness. The effect ofbanding can be smoothed by the dispersion typically exhibited bybroadband sources such as LEDs. However, LED illumination is notentirely free from the banding problem and, moreover, tends to result inbulky input optics and an increase in the thickness of the waveguide.Debanding can be minimized using a pupil shifting technique forconfiguring the light coupled into the waveguide such that the inputgrating has an effective input aperture that is a function of the TIRangle. Techniques for performing pupil-shifting in internationalapplication No. PCT/US2018/015553 entitled “Waveguide Device withUniform Output Illumination,” the disclosure of which is herebyincorporated by reference in its entirety.

In some cases, the polarization rotation that takes place in foldgratings (described above) can compensate for illumination banding inwaveguides that uses laser illumination. The mechanism for this is thatthe large number of grating interactions in a fold grating combined withthe small polarization rotation at each interaction can average out thebanding (arising from imperfect matching of TIR beams and other coherentoptical effects such as but not limited to those arising from parasiticgratings left over from the recording process, stray light interactionswith the grating and waveguide surfaces, etc.). The process ofcompensating for the birefringence can be aided by fine tuning thespatial variation of the birefringence (alignment of the LC directors)in the fold grating.

A further issue that arises in waveguide displays is that contact withmoisture or surface combination can inhibit waveguide total internalreflection (TIR), leading to image gaps. In such cases, the scope forusing protective outer layers can be limited by the need for low indexmaterials that will provide TIR over the waveguide angular bandwidth. Afurther design challenge in waveguides is maintaining high efficiencyover the angular bandwidth of the waveguide. One exemplary solutionwould be a polarization-maintaining, wide-angle, and high-reflectionwaveguide cladding. In some applications, polarization balancing withina waveguide can be accomplished using either a quarter wave retardinglayer or a half wave retarder layer applied to one or both of theprincipal reflecting surfaces of the waveguide. However, in some cases,practical retarder films can add unacceptable thickness to thewaveguide. Thin film coatings of the required prescription will normallyentail an expensive and time-consuming vacuum coating step. Oneexemplary method of implementing a coating includes but not limited tothe use of an inkjet printing or industry-standard spin-coatingprocedure. In many embodiments, the coating could be applied directly toa printed grating layer. Alternatively, the coating could be applied toan external optical surface of the assembled waveguide.

In some applications, waveguides are combined with conventional opticsfor correcting aberrations. Such aberrations may arise when waveguidesare used in applications such as but not limited to a car HUD, whichprojects an image onto a car windscreen for reflection into the viewer'seyebox. The curvatures of the windscreen can introduce significantgeometric aberration. Since many waveguides operate with collimatedbeams, it can be difficult to pre-compensate for the distortion withinthe waveguide itself. One solution includes mounting a pre-compensatingoptical element near the output surface of the waveguides. In manycases, the optical element is molded in plastic and can introduce severebirefringence, which should be balanced by the waveguide.

In view of the above, many embodiments of the invention are directedtowards birefringence control layers designed to address one or more ofthe issues posed above. For example, in many embodiments, a compact andefficient birefringence control layer is implemented for compensatingfor polarization related losses in holographic waveguides, for providingthree-dimensional LC director alignment in waveguides based on Bragggratings, for spatially varying angular/spectral bandwidth forhomogenizing the output from a waveguide, and/or for isolating awaveguide from its environment while ensuring confinement of wave-guidedbeams. In some embodiments, a polarization-maintaining, wide-angle, andhigh-reflection waveguide cladding with polarization compensation isimplemented for grating birefringence. In several embodiments, a thinpolarization control layer is implemented for providing either quarterwave or half wave retardation. A polarization control layer can beimplemented as a thin layer directly on top of the grating layer or toone or both of the waveguide substrates using a standard spin coating orinkjet printing process. In a number of embodiments, apolarization-maintaining, wide-angle birefringence control layer isimplemented for modifying the polarization output of a waveguide tobalance the birefringence of an external optical element used with thewaveguide. Other implementations and specific configurations arediscussed below in further detail.

Waveguide Applications Incorporating Birefringence Control

Waveguides and waveguide displays implementing birefringence controltechniques in accordance with various embodiments of the invention canbe achieved using many different techniques. In some embodiments, thewaveguide includes a birefringent grating layer and a birefringencecontrol layer. In further embodiments, a compact and efficientbirefringence control layer is implemented. A birefringence controllayer can be implemented for various functions such as but not limitedto: compensating for polarization related losses in holographicwaveguides; providing three-dimensional LC director alignment inwaveguides based on Bragg gratings; and efficient and cost-effectiveintegration within a waveguide for spatially varying angular/spectralbandwidth for homogenizing the output from the waveguide. In any of theembodiments to be described, the birefringence control layer may beformed on any optical surface of the waveguide. For the purposes ofunderstanding the invention, an optical surface of the waveguide may beone of the TIR surfaces, a surface of the grating layer, a surface ofthe waveguide substrates sandwiching the grating layer, or a surface ofany other optical substrate implemented within the waveguide (forexample, a beam-splitter layer for improving uniformity).

FIG. 1 conceptually illustrates a waveguide implementing a birefringencecontrol layer in accordance with an embodiment of the invention. In theillustrative embodiment, the waveguide apparatus 100 includes an opticalsubstrate 101 containing a birefringent grating layer 102 and abirefringence control layer 103. As shown, light 104 propagating underTIR within the waveguide interacts with both layers. For example, thelight ray 104A with an initial polarization state represented by thesymbol 104B has its polarization rotated to the state 104C afterpropagation through the grating region around the point 102A. Thebirefringence control layer 103 rotates the polarization vector into thestate 104D, which is the polarization state for achieving somepredefined diffraction efficiency of the ray 104E when it interacts withthe grating around the point 102B and is diffracted into the direction104F with a polarization state 104G, which is similar to the state 104D.As will be shown in the following description, many differentconfigurations of a birefringence control layer and birefringent gratingcan be implemented in accordance with various embodiments of theinvention.

FIG. 2 conceptually illustrates a waveguide apparatus 200 that includesat least one optical substrate 201 and a coupler 202 for deflectinglight 203A, 203B (covering a range of incident angles) from an externalsource 204 into TIR paths 205A, 205B in the waveguide substrate. Lightin the TIR path can interact with the output grating, which can beconfigured to extract a portion of the light each time the TIR lightsatisfies the condition for diffraction by the grating. In the case of aBragg grating, extraction can occur when the Bragg condition is met.More precisely, efficient extraction can occur when a ray incident onthe grating lies within an angular bandwidth and spectral bandwidtharound the Bragg condition. The bandwidths being defined according tosome measure of acceptable diffraction efficiency (such as but notlimited to 50% of peak DE). For example, light in the TIR ray paths205A, 205B is diffracted by the output grating into output direction206A, 206B, 207A, and 207B at different points along the output grating.It should be apparent from basic geometrical optics that a unique TIRangle can be defined by each light incidence angle at the input grating.

Many different types of optical elements can be used as the coupler. Forexample, in some embodiments, the coupler is a grating. In severalembodiments, the coupler is a birefringent grating. In many embodiments,the coupler is a prism. The apparatus further includes at least onebirefringent grating 208 for providing beam expansion in a firstdirection and light extraction from the waveguide and at least onebirefringence control layer 209 with anisotropic refractive indexproperties. In the embodiments to be discussed, the source 204 can be aninput image generator that includes a light source, a microdisplaypanel, and optics for collimating the light. As can readily beappreciated, various input image generators can be used, including thosethat output non-collimated light. In many embodiments, the input imagegenerator projects the image displayed on the microdisplay panel suchthat each display pixel is converted into a unique angular directionwithin the substrate waveguide. The collimation optics may include lensand mirrors, which can be diffractive lenses and mirrors. In someembodiments, the source may be configured to provide illumination thatis not modulated with image information. In several embodiments, thelight source can be a laser or LED and can include one or more lensesfor modifying the illumination beam angular characteristics. In a numberof embodiments, the image source can be a micro-display or an imagescanner.

The interaction of the light with the birefringence control layer 209and the birefringent grating 208 integrated along the total internalreflection path for any direction of the light can provide a predefinedcharacteristic of the light extracted from the waveguide. In someembodiments, the predefined characteristic includes at least one of auniform polarization or a uniform illumination over the angular range ofthe light. FIG. 2 also illustrates how the birefringence control layer209 and grating 208 provide uniform polarization. In many embodiments,the input state will correspond to P polarization, a state which may beused for gratings recorded in HPDLC. For the purposes of explaining theinvention, an initial polarization state represented by 210 is assumed.The interaction of the light with the birefringence control layer near agrating interaction region along the TIR path 205A is represented by thepolarization states 211, 212, which show the rotation of thepolarization vector before and after propagation through the thicknessAB of the birefringence control layer 209. This polarization rotationcan be designed to balance the polarization rotation through thethickness CD of the adjacent grating region the ray encounters along theTIR path 205A. Thus, the polarization of the light extracted by thegrating can be aligned parallel to the input polarization vector asindicated by the polarization state 213. In some embodiments, the outputpolarization state may differ from the input polarization state. In anumber of embodiments, such as the one shown in FIG. 2 , there is atleast partial overlap of the birefringent grating and the birefringencecontrol layer. In several embodiments, the two are separated by aportion of the waveguide path.

FIG. 3 conceptually illustrates a waveguide apparatus 300 in which thebirefringence control layer and grating provide uniform outputillumination in accordance with an embodiment of the invention. In theillustrative embodiment, the waveguide apparatus 300 includes at leastone optical substrate 301 and a coupler 302 for deflecting light 303from an external source 304 into TIR path 305 in the waveguidesubstrate. The apparatus 300 further includes at least one birefringentgrating 306 for providing beam expansion in a first direction and lightextraction from the waveguide and at least one birefringence controllayer 307 with anisotropic index properties. As shown, light in the TIRray paths 305 can be diffracted by the output grating into outputdirection 308, 309. For the purposes of explaining the invention, aninitial beam illumination (I) versus angle (U) profile represented by310 is assumed. The interaction of the light with the birefringencecontrol layer 307 near a grating interaction region along the TIR path305 is characterized by the illumination profiles before (311) and after(312) propagation through the thickness AB of the birefringence controllayer. In some applications, such as but not limited to displayapplications, the waveguide apparatus 300 can be designed to haveuniform illumination versus angle across the exit pupil of thewaveguide. This may be achieved by matching the birefringence versusangle characteristics of the birefringence control layer to the angularbandwidth of the grating (along nearby grating paths CD in proximity tothe path AB) such that the light extracted by the grating (indicated by308, 309) integrated across the waveguide exit pupil provides uniformillumination versus angle distribution 313. In some embodiments, thecharacteristics of the grating and birefringence control layer vary overthe aperture of the waveguide.

Implementing Birefringence Control Layers

Various materials and fabrication processes can be used to provide abirefringence control layer. In many embodiments, the birefringentcontrol layer has anisotropic index properties that can be controlledduring fabrication to provide a spatial distribution of birefringencesuch that the interaction of the light with the birefringence controllayer and the birefringent grating integrated along the total internalreflection path for any direction of the light provides a predefinedcharacteristic of the light extracted from the waveguide. In someembodiments, the layer may be implemented as a thin stack that includesmore than one layer.

Alignment of HPDLC gratings can present significant challenges dependingon the grating configuration. In the simplest case of a plane grating,polarization control can be confined to a single plane orthogonal to thegrating plane. Rolled K-vector gratings can require the alignment tovary across the grating plane. Fold gratings, particularly ones withslanted Bragg fringes, can have much more complicated birefringence,requiring 3D alignment and, in some cases, more highly spatiallyresolved alignment.

The following examples of birefringence control layers for use with theinvention are illustrative only. In each case, it is assumed that thelayer is processed such that the properties vary across the surface ofthe layer. It is also assumed that the birefringence control layer isconfigured within the waveguide or on an optical surface of thewaveguide containing the grating. In some embodiments, the birefringencecontrol layer is in contact with the grating layer. In several cases,the birefringence control layer spits into separate sections and aredisposed on different surfaces of the waveguide. In a number ofembodiments, a birefringence layer may include multiple layers.

In some embodiments, the invention provides a thin polarization controllayer that can provide either quarter wave or half wave retardation. Thepolarization control layer can be implemented as a thin layer directlyon top of the grating layer or to one or both of the waveguidesubstrates using a standard spin coating or ink jet printing process.

In one group of embodiments, the birefringence control layer is formedusing materials using liquid crystal and polymer networks that can bealigned in 3D using directional UV light. In some embodiments, thebirefringence control layer is formed at least in part from a LiquidCrystal Polymer (LCP) Network. LCPs, which have also been referred to inthe literature as reactive mesogens, are polymerizable liquid crystalscontaining liquid crystalline monomers that include, for example,reactive acrylate end groups, which polymerize with one another in thepresence of photo-initiators and directional UV light to form a rigidnetwork. The mutual polymerization of the ends of the liquid crystalmolecules can freeze their orientation into a three-dimensional pattern.The process typically includes coating a material system containingliquid crystal polymer onto a substrate and selectively aligning the LCdirectors using directionally/spatially controllable UV source prior toannealing. In some embodiments, the birefringence control layer isformed at least in part from a Photo-Alignment Layer, also referred toin the literature as a linearly polymerized photopolymer (LPP). An LPPcan be configured to align LC directors parallel or perpendicular toincident linearly polarized UV light. LPP can be formed in very thinlayers (typically 50 nm) minimizing the risks of scatter or otherspurious optical effect. In some embodiments, the birefringence controllayer is formed from LCP, LPP, and at least one dopant. Birefringencecontrol layers based on LCPs and LPPs can be used align LC directors inthe complex three-dimensional geometries characteristic of fold gratingsand rolled K-vector gratings formed in thin film (2-4 microns). In someembodiments, a birefringence control layer based on LCPs or LPPs furtherincludes dichroic dyes, chiral dopants to achieve narrow or broadbandcholesteric filters, twisted retarders, or negative c-plate retarders.In many embodiments, birefringence control layers based on LCPs or LPPsprovide quarter or half-wave retardation layers.

In some embodiments, the birefringence control layer is formed by amultilayer structure combining isotropic and anisotropic index layers(as shown in FIG. 4 ). In FIG. 4 , the multilayer structure 400 includesisotropic layers 401, 402 and anisotropic index layers 403, 404. In someembodiments, a multiplayer stack may include a high number of layers,such as but not limited to several tens or several hundreds of layers.FIG. 5 conceptually illustrates a multilayer structure 500 that includesisotropic layers 501, 502 and anisotropic index layers 503, 504 combinedwith a birefringent grating layer 505. When birefringence is on theorder of the change of the in-plane refractive index between adjacentmaterial layers of the stack, it is possible to achieved improvedcontrol of the reflectivity of P-polarized light. Normally in isotropicmaterials Brewster's law dictates that for any interface, there is anangle of incidence (Brewster's angle) for which the P-polarizationreflectivity vanishes. However, the reflectivity can increasedramatically at other angles. The limitations imposed by the Brewsterangle can be overcome by applying the basic principles discussed inWeber et al., “Giant Birefringent Optics in Multilayer Polymer Mirrors,”published in Science, Vol. 287, 31 Mar. 2000, pages 2451-2456. Becausethe optical characteristic of systems of isotropic/anisotropic indexlayers are based on the fundamental physics of interfacial reflectionand phase thickness and not on a particular multilayer interferencestack design, new design freedoms are possible. Designs for wide-angle,broadband applications are simplified if the Brewster angle restrictionis eliminated, particularly for birefringence control layers immersed ina high-index medium such as a waveguide substrate. A further advantagein relation to waveguide displays is that color fidelity can bemaintained for all incidence angles and polarizations.

A birefringent grating will typically have polarization rotationproperties that are functions of angle wavelength. The birefringencecontrol layer can be used to modify the angular, spectral, orpolarization characteristics of the waveguide. In some embodiments, theinteraction of light with the birefringence control layer can provide aneffective angular bandwidth variation along the waveguide. In manyembodiments, the interaction of light with the birefringence controllayer can provide an effective spectral bandwidth variation along thewaveguide. In several embodiments, the interaction of light with thebirefringence control layer can provide a polarization rotation alongthe waveguide. In a number of embodiments, the grating birefringence canbe made to vary across the waveguide by spatially varying thecomposition of the liquid crystal polymer mixture during gratingfabrication. In some embodiments, the birefringence control layer canprovide a birefringence variation in at least one direction in the planeof the waveguide substrate. The birefringence control layer can alsoprovide a means for optimizing optical transmission (for differentpolarizations) within the waveguide. In many embodiments, thebirefringence control layer can provide a transmission variation in atleast one direction in the plane of the waveguide substrate. In severalembodiments, the birefringence control layer can provide an angulardependence of at least one of beam transmission or polarization rotationin at least one direction in the plane of the waveguide substrate. In anumber of embodiments, the birefringence control layer can provide aspectral dependence of at least one of beam transmission or polarizationrotation in at least one direction in the plane of the waveguidesubstrate.

In many embodiments, birefringent gratings may provide input couplers,fold gratings, and output gratings in a wide range of waveguidearchitectures. FIG. 6 conceptually illustrates a plan view of a dualexpansion waveguide with birefringent control layers in accordance withan embodiment of the invention. In the illustrative embodiment, thewaveguide 600 includes an optical substrate 601 that contains an inputgrating 602, a fold grating 603, and an output grating 604 that areoverlaid by polarization control layers 605, 606, 607, respectively.

In some embodiments, the invention provides a polarization-maintaining,wide angle birefringence control layer for modifying the polarizationoutput of a waveguide to balance the birefringence of an externaloptical element used with the waveguide. FIG. 7 conceptually illustratesan embodiment of the invention directed at automobile HUDs, whichreflect collimated imagery off the windscreen into an eyebox. Anywindscreen curvature will typically result in aberrations and othergeometrical distortion, which cannot be corrected in certain waveguideimplementations with the requirement for the beam to remainsubstantially collimated. One solution to this problem is to mount acorrection element, which may be a conventional refractive element or adiffractive element, near the output surface of the waveguide. In suchimplementations, the birefringence correction component can avoiddisturbing ray paths from the waveguide and can be achromatic. Thecompensator technology used can provide spatially-varying configuration,low haze, and high transmission. In the illustrative embodiment of FIG.7 , the waveguide 700 includes an optical substrate 701 containing agrating coupler 702 for deflecting light 703 from an external source ofimage modulated light (not shown) into the TIR path 704 in thewaveguide, a birefringent grating 705 for providing beam expansion in afirst direction and extracting light from the waveguide, and abirefringence control layer 706. The apparatus 700 further includes anoptical element 707 disposed in proximity to the waveguide forcorrecting geometrical distortions and other aberrations introduced byreflection at the windscreen. In some embodiments, the optical element707 is a refractive lens. In other embodiments, the optical element 707can be a diffractive lens. For wide field of view HUDs providing agenerous eye box, the corrector will typically have a large footprintwith a horizontal dimension (along the dashboard) as large as 400 mm.However, if the corrector is molded in plastic, it will tend to sufferfrom birefringence. Hence, in the embodiment of FIG. 7 , thebirefringence control element 706 can be designed to compensate for boththe grating polarization and polarization rotation introduced by theoptical element 707. Referring again to FIG. 7 , an initial polarizationstate corresponding to P polarization is assumed. The polarization stateafter propagation through the birefringence grating, birefringencecontrol layer, and the correction elements is represented by the symbols708-711. The interaction of the light with the birefringence controllayer near to a grating interaction region along the TIR path isrepresented by the polarization states. In the embodiment of FIG. 7 ,the polarization of the light 712, 713 extracted by the grating isaligned parallel to the input polarization vector. In some embodiments,the birefringence control layer 706 may be configured to rotate theoutput light polarization vector through ninety degrees.

In some embodiments, the birefringence control layer can be provided byvarious techniques using mechanical, thermal, or electro-magneticprocessing of substrates. For example, in some embodiments, thebirefringence control layer is formed by applying spatially varyingmechanical stress across the surface of an optical substrate. FIG. 8conceptually illustrates an apparatus 800 for aligning a birefringencecontrol layer 801 in which forces are applied in the directionsindicated by 802-805, resulting in the iso-birefringence contours 806.In many embodiments, the forces illustrated do not necessarily all needto be applied to the layer. In some embodiments, the birefringencecontrol layer 801 is formed by inducing thermal gradients into anoptical substrate. In a number of embodiments, the birefringence controllayer 801 is provided by a HPDLC grating in which LC directors arealigned using electric or magnetic fields during curing. In severalembodiments, two or more of the above techniques may be combined.

Fabrication of Waveguides Implementing Birefringence Control Layers

The present invention also provides methods and apparatus forfabricating a waveguide containing a birefringent grating and abirefringence control layer. The construction and arrangement of theapparatus and methods as shown in the various exemplary embodiments areillustrative only. Although only a few embodiments have been describedin detail in this disclosure, many modifications are possible (forexample, additional steps for improving the efficiency of the processand quality of the finished waveguide, minimizing process variances,monitoring the process and others.) Any process step referring to theformation of a layer should be understood to cover multiple such layers.For example, where a process step of recording a grating layer isdescribed, this step can extend to recording a stack containing two ormore grating layers. Accordingly, all such modifications are intended tobe included within the scope of the present disclosure. The order orsequence of any process or method steps may be varied or re-sequencedaccording to alternative embodiments. Other substitutions,modifications, changes, and omissions may be made in the design of theprocess apparatus, operating conditions and arrangement of the exemplaryembodiments without departing from the scope of the present disclosure.For the purposes of explaining the invention, the description of theprocesses will refer to birefringence control layers based on liquidcrystal polymer material systems as described above. However, it shouldbe clear from the description that the processes may be based on any ofthe implementations of a birefringence control layer described herein.

FIGS. 9A-9F conceptually illustrate the process steps and apparatus forfabricating a waveguide containing a birefringent grating and abirefringence control layer in accordance with various embodiments ofthe invention. FIG. 9A shows the first step 900 of providing a firsttransparent substrate 901. FIG. 9B illustrates an apparatus 910 forapplying holographic recording material to the substrate 901. In theillustrative embodiment, the apparatus 910 includes a coating apparatus911 that provides a spray pattern 912 that forms a layer 913 of gratingrecording material onto the substrate 901. In some embodiments, thespray pattern may include a narrow jet or blade swept or stepped acrossthe surface to be coated. In several embodiments, the spray pattern mayinclude a divergent jet for covering large areas of a surfacesimultaneously. In a number of embodiments, the coating apparatus may beused in conjunction with one or more masks for providing selectivecoating of regions of the surface. In many embodiments, the coatingapparatus is based on industry-standard standard spin-coating or ink-jetprinting processes.

FIG. 9C conceptually illustrates an apparatus 920 for exposing a layerof grating recording material to form a grating layer in accordance withan embodiment of the invention. In the illustrative embodiment, theapparatus 920 contains a master grating 921 for contact copying thegrating in the recording material and a laser 922. As shown, the master921 diffracts incident light 923 to provide zero order 924 anddiffracted light 925, which interferes within the grating material layerto form a grating layer 926. The apparatus may have further features,such as but not limited to light stops and masks for overcoming straylight from higher diffraction orders or other sources. In someembodiments, several gratings may be recorded into a single layer usingthe principles of multiplexed holograms. FIG. 9D conceptuallyillustrates an apparatus 930 for coating a layer of liquid crystalpolymer material onto the grating layer in accordance with an embodimentof the invention. In the illustrative embodiment, the apparatus 930contains a coating apparatus 931 configured to deliver a spray pattern932 forming a layer of material 933. The coating apparatus 931 may havesimilar features to the coating apparatus used to apply the gratingrecording material. FIG. 9E conceptually illustrates an apparatus 940for providing an aligned liquid crystal polymer layer of material inaccordance with an embodiment of the invention. In the illustrativeembodiment, the apparatus 940 contains a UV source (which can includecollimation, beams steering, and beam shaping optics, depending on thespecific requirements of a given application) 941 providing directionalUV light 942 for forming an aligned LC polymer layer 943. FIG. 9Fconceptually illustrates the completed waveguide 950 after the step ofapplying a second substrate 951 over the aligned liquid crystal polymerlayer 943.

In some embodiments, exposure of the grating recording material may useconventional cross beam recording procedures instead of the masteringprocess described above. In many embodiments, further processing of thegrating layer may include annealing, thermal processing, and/or otherprocesses for stabilizing the optical properties of grating layer. Insome embodiments, electrodes coatings may be applied to the substrates.In many embodiments, a protective transparent layer may be applied overthe grating layer after exposure. In a number of embodiments, the liquidcrystal polymer material is based on the LCP, LPP material systemsdiscussed above. In several embodiments, the alignment of the liquidcrystal polymer can result in an alignment of the liquid crystaldirectors parallel to the UV beam direction. In other embodiments, thealignment is at ninety degrees to the UV beam direction. In someembodiments, the second transparent substrate may be replaced by aprotective layer applied using a coating apparatus.

FIGS. 10A-10F conceptually illustrate the process steps and apparatusfor fabricating a waveguide containing a birefringent grating with abirefringence control layer applied to an outer surface of the waveguidein accordance with various embodiments of the invention. FIG. 10Aconceptually illustrates the first step 1000 of providing a firsttransparent substrate 1001 in accordance with an embodiment of theinvention. FIG. 10B conceptually illustrates an apparatus 1010 forapplying holographic recording material to the substrate in accordancewith an embodiment of the invention. In the illustrative embodiment, theapparatus 1010 includes a coating apparatus 1011 providing a spraypattern 1012 that forms the layer 1013 of grating recording materialonto the substrate 1001. FIG. 10C conceptually illustrates an apparatus1020 for exposing a layer of grating recording material to form agrating layer in accordance with an embodiment of the invention. In theillustrative embodiment, the apparatus 1020 includes a master grating1021 for contact copying the grating in the recording material and alaser 1022. As shown, the master 1021 converts light 1023 from the laser1022 into zero order 1024 and diffracted light 1025, which interferewithin the grating material layer 1013 to form a grating layer 1026.FIG. 10D conceptually illustrates the partially completed waveguide 1030after the step of applying a second substrate 1031 over the exposedgrating layer in accordance with an embodiment of the invention. FIG.10E conceptually illustrates an apparatus 1040 for coating a layer ofliquid crystal polymer material onto the second substrate in accordancewith an embodiment of the invention. In the illustrative embodiment, theapparatus 1040 includes a spray coater 1041 for delivering a spraypattern 1042 to form a layer of material 1043. FIG. 10F conceptuallyillustrates an apparatus 1050 for aligning the liquid crystal polymermaterial in accordance with an embodiment of the invention. In theillustrative embodiment, the apparatus 1050 includes a UV source 1051providing the directional UV light 1052 for forming an aligned liquidcrystal polymer layer 1053, which can be configured to realign the LCdirectors of the grating layer 1026.

FIGS. 11A-11F conceptually illustrate the process steps and apparatusfor fabricating a waveguide containing a birefringent grating and abirefringence control layer in accordance with various embodiments ofthe invention. Unlike the above described embodiments, the step offorming the birefringence control layer can be carried out before therecording of the grating layer, which is formed above the birefringencecontrol layer. FIG. 11A conceptually illustrates the first step 1100 ofproviding a first transparent substrate 1101. FIG. 11B conceptuallyillustrates an apparatus 1110 for coating a layer of liquid crystalpolymer material onto the first substrate in accordance with anembodiment of the invention. In the illustrative embodiment, theapparatus 1110 includes a coating apparatus 1111 configured to deliver aspray pattern 1112 to form a layer of material 1113. FIG. 11Cconceptually illustrates an apparatus 1120 for aligning the liquidcrystal polymer material in accordance with an embodiment of theinvention. In the illustrative embodiment, the apparatus 1120 includes aUV source 1121 providing the directional UV light 1122 for forming analigned liquid crystal polymer layer 1123. FIG. 11D conceptuallyillustrates an apparatus 1130 for applying holographic recordingmaterial to the substrate in accordance with an embodiment of theinvention. In the illustrative embodiment, the apparatus 1130 includes acoating apparatus 1131 for providing a spray pattern 1132 to form alayer of grating recording material 1133 on top of the liquid crystalpolymer layer 1123. FIG. 11E conceptually illustrates an apparatus 1140for exposing a layer of grating recording material to form a gratinglayer in accordance with an embodiment of the invention. In theillustrative embodiment, the apparatus 1140 includes a master grating1141 for contact copying the grating in the recording material and alaser 1142. As shown, the master 1141 converts light 1142 from the laserinto zero order 1143 and diffracted light 1144, which interfere in thegrating material layer 1133 to form a grating layer 1145, which isaligned by the liquid crystal polymer material layer 1123. FIG. 11Fconceptually illustrates the completed waveguide 1150 after the step ofapplying a second substrate 1151 over the exposed grating layer inaccordance with an embodiment of the invention.

FIG. 12 conceptually illustrates a flow chart illustrating a method offabricating a waveguide containing a birefringent grating and abirefringence control layer in accordance with an embodiment of theinvention. Referring to FIG. 12 , the method 1200 includes providing(1201) a first transparent substrate. A layer of grating recordingmaterial can be deposited (1202) onto the substrate. The layer ofgrating recording material can be exposed (1203) to form a gratinglayer. A layer of liquid crystal polymer material can be deposited(1204) onto the grating layer. The liquid crystal polymer material canbe aligned (1205) using directional UV light. A second transparentsubstrate can be applied (1206) over the alignment layer.

FIG. 13 conceptually illustrates a flow chart illustrating a method offabricating a waveguide containing a birefringent grating and abirefringence control layer applied to an outer surface of the waveguidein accordance with an embodiment of the invention. Referring to FIG. 13, the method 1300 includes providing (1301) a first transparentsubstrate. A layer of grating recording material can be deposited (1302)onto the substrate. The layer of grating recording material can beexposed (1303) to form a grating layer. A second transparent substratecan be applied (1304) over the exposed grating layer. A layer of liquidcrystal polymer material can be deposited (1305) onto the secondtransparent substrate. The liquid crystal polymer material can bealigned (1306) using directional UV light.

FIG. 14 conceptually illustrates a flow chart illustrating a method offabricating a waveguide containing a birefringent grating and abirefringence control layer where forming the birefringence controllayer is carried out before the recording of the grating layer inaccordance with an embodiment of the invention. Referring to FIG. 14 ,the method 1400 includes providing (1401) a first transparent substrate.A layer of liquid crystal polymer material can be deposited (1402) ontothe substrate. The liquid crystal polymer material can be aligned (1403)using directional UV light. A layer of grating recording material can bedeposited (1404) onto the aligned liquid crystal polymer material. Thelayer of grating recording material can be exposed (1405) to form agrating layer. A second transparent substrate can be applied (1406) overthe grating layer.

Although FIGS. 12-14 illustrate specific processes for fabricatingwaveguides containing a birefringent grating and a birefringence controllayer, many other fabrication processes and apparatus can be implementedto form such waveguides in accordance with various embodiments of theinvention. For example, the order or sequence of any process or methodsteps may be varied or re-sequenced according to alternativeembodiments. Other substitutions, modifications, changes, and omissionsmay be made in the design of the process apparatus, operating conditionsand arrangement of the exemplary embodiments without departing from thescope of the present disclosure.

Additional Embodiments and Applications

In some embodiments, a polarization-maintaining, wide angle, highreflection waveguide cladding with polarization compensation for gratingbirefringence can be implemented. FIG. 15 shows one such embodiment. Inthe illustrative embodiment, the waveguide 1500 includes a waveguidingsubstrate 1501 containing a birefringent grating 1502 and abirefringence control layer 1503 overlaying the waveguiding substrate1501. As shown, guided light 1504 interacting with the birefringencecontrol layer 1503 at its interface with the waveguiding substrate 1501has its polarization rotated from the state indicated by symbol 1505(resulting from the previous interaction with the grating) to the stateindicated by 1506 (which has a desired orientation for the nextinteraction with the grating, for example, having an orientation forproviding a predefined diffraction efficiency at some pre-defined pointalong the grating).

In many embodiments, a compact and efficient birefringence control layerfor isolating a waveguide from its environment while ensuring efficientconfinement of wave-guided beams can be implemented. FIG. 16 illustratesone such embodiment. In the illustrative embodiment, the environmentallyisolated waveguide 1600 includes a waveguiding substrate 1601 containinga birefringent grating 1602 and a birefringence control layer 1603overlaying the waveguiding substrate 1601. As shown, guided light 1604interacting with the birefringence control layer 1603 at its interfacewith the waveguiding substrate 1601 has its polarization rotated fromthe state indicated by the symbol 1605 to the state indicated by 1606.Environmental isolation of the waveguide can be provided by designingthe birefringence control layer 1603 such that total internal reflectionoccurs at the interface 1607 between the birefringence control layer1603 and the waveguiding substrate 1601. In some embodiments,environmental isolation is provided by designing the birefringencecontrol layer to have gradient index characteristics such that only asmall portion of the guided light is reflected at the air interface ofthe birefringence control layer. In several embodiments, thebirefringence control layer may incorporate a separate GRIN layer. In anumber of embodiments, a GRIN layer may be based on embodimentsdisclosed in U.S. Provisional Patent Application No. 62/123,282 entitledNEAR EYE DISPLAY USING GRADIENT INDEX OPTICS and U.S. Provisional PatentApplication No. 62/124,550 entitled WAVEGUIDE DISPLAY USING GRADIENTINDEX OPTICS.

FIG. 17 conceptually illustrates an apparatus 1700, which may be used inconjunction with some of the methods described above, for fabricating astructure containing a birefringent grating layer 1701 overlaying abirefringence control layer 1702 in accordance with an embodiment of theinvention. In FIG. 17 , the substrate supporting the birefringencecontrol layer is not shown. The construction beams, indicated by rays1703, 1704, may be provided by a master grating or a crossed beamholographic recording setup. As shown, the construction beams propagatethrough the birefringence control layer 1702. In many embodiments, theconstruction beams are in the visible band. In some embodiments, theconstruction beams are in the UV band.

FIG. 18 conceptually illustrates an apparatus 1800, which may be used inconjunction with some of the methods described above, for fabricating astructure containing a birefringence control layer 1801 overlaying abirefringent grating layer 1802 in accordance with an embodiment of theinvention. In FIG. 18 , the substrate supporting the grating layer isnot shown. The direction of a recording beam is indicated by 1803. Inmany embodiments the birefringence control layer is a liquid crystalpolymer material system which uses a directional UV beam for alignment.In some embodiments, in which the grating is a recorded in a polymer andliquid crystal material system, an exposed grating may be erased duringthe process of aligning the birefringence control layer by applying anexternal stimulus such as heat, electric or magnetic fields or light toeffective to create an isotropic phase of the liquid crystal.

FIG. 19 conceptually illustrates a cross section of waveguide 1900containing substrates 1901, 1902 sandwiching a grating layer 1903. Asshown, a source 1904 emits collimated light 1905A, which is coupled bythe grating layer into the total internal reflection (TIR) pathindicated by rays 1905B, 1905C and extracted by the grating layer 1903into the output ray path 1905D. In the illustrative embodiments, thesource 1904 can be a variety of light sources, including but not limitedto a laser or a LED.

FIG. 20 conceptually illustrates a waveguide similar to the one of FIG.19 with a quarter wave polarization layer inserted by replacing thesubstrate 1902 by a quarter wave film 2001 sandwiched by substrates2002, 2003 in accordance with an embodiment of the invention. A quarterwave polarization layer can be beneficial to the holographic waveguidedesign in two ways. Firstly, it can reduce reinteraction (outcoupling)of a rolled K-vector (RKV) input grating to increase the overallcoupling efficiency of the input grating. Secondly, it can continuouslymix up the polarization of the light going into the fold and outputgratings to provide better extraction. The quarter wave layer can belocated on a waveguide surface along the optical from the input grating.Typically, a waveguide surface can include one of the TIR surface of thewave or some intermediate surface formed inside the waveguide. Theoptical characteristics of the quarter wave layer can be optimized for“waveguide angles”—i.e., angles in glass beyond the TIR angle. In someembodiments, the center field is designed at approximately 55 deg. inglass (corresponding to a refractive index of approximately 1.51 atwavelength 532 nm). In many embodiments, optimization for red, green,and blue can be used for optimum performance of red, green, and bluetransmitting waveguides. As will be shown in the embodiments to bedescribed, there are several different ways of incorporating the quarterwave film within a waveguide. In the following embodiment, we refergenerally to a quarter wave polarization layer provided by a liquidcrystal polymer (LCP) material. However, it should be understood thatother materials be used in applications of the invention.

FIG. 21 conceptually illustrates a schematic cross section view 2100showing a portion of a waveguide illustrating how the use of a quarterwave polarization layer with a RKV grating can overcome the problem ofunwanted extraction of light along the propagation path in the inputgrating portion of the waveguide in accordance with an embodiment of theinvention. One ray path is illustrated in which input light includingthe P-polarized light 2101A is coupled by the grating layer into a TIRpath indicated by the rays 2101B-2101L in the waveguide. The waveguidegrating has rolled K-vectors of which examples occurring at three pointsalong the length of the waveguide are illustrated schematically by thevectors 2102A-2102C. In the illustrative embodiment, the light 2101Adiffractively coupled into TIR by the input grating is P-polarized withrespect to the grating. In many embodiments, the TIR angle can benominally 55 degrees in glass. On transmission through the quarter wavelayer, the polarization of the light changes from P to circularlypolarized (2101C). After TIR at the lower surface of the waveguide thepolarization changes to circularly polarized light (2101D) of anopposing sense such that after traversing the quarter wave layer on itsupward path becomes S-polarized (2101E) with respect to the grating. TheS-polarized light passes through the gating without deviation (2101F) orsubstantial loss since it is both off-Bragg and “off polarization”(since the grating has zero or low diffraction efficiency for S). Thislight then undergoes TIR (2101G) a second time retaining itsS-polarization. Hence the light 2101G is now on-Bragg but is still offpolarization with respect to the P-polarization sensitive grating. Thelight therefore passes through the grating without diffraction (2101H).At this location the RKV (2102B) has rolled slightly from the one(2102A) near the light entry point on the UP grating. If the light was“on polarization,” the ‘roll’ effect of RKV would be small, and so thelight would be strongly out-coupled. The S-polarization light passingthrough the grating goes through another full cycle, (2101H-2101M) in asimilar fashion to the cycle illustrated by rays 2101B-2101G, and thenreturns to a P-polarized state for the next (2101M) on-Bragg interactionat the grating region with K-vector 2102C. At this point, the light hasperformed two complete TIR bounce cycles down the waveguide, increasingthe angular separation of the incidence angle at the grating andK-vector, which strongly reduces the on-Bragg interaction.

To clarify the embodiment of FIG. 21 further, a 55-degree TIR anglelight in a 1 mm thick waveguide is considered, with a 20 mm projectorrelief (distance of the projector from the input grating), and a nominal4.3 mm diameter projector exit pupil: The first interaction with thegrating takes place approximately 2.85 mm down the waveguide. Thisequates to an 8.1-degree angle at 20 mm projector relief. For comparisonthe FWHM angular bandwidth of a typical 1.6 um grating is about 12degrees in air (prescription dependent) i.e. the angle subtended by thepupil is not much larger than the semi-width of the lens. This leads tostrong outcoupling if polarization is not changed to S-polarized asdescribed above. In effect, the use of the quarter wave layer doublesthe TIR length to approximately 5.7 mm. This offset equates to about15.9 deg, which is larger than the angular bandwidth of most waveguidegratings, thereby reducing outcoupling reinteraction losses from thewaveguide.

FIG. 22 conceptually illustrates a polarization layer architecture 2200containing an LCP quarter wave cell and a reactive monomer liquidcrystal mixture (RMLCM) cell separated by index matching oil layer(2201) in accordance with an embodiment of the invention. The LCP cellincludes a substrate 2202 and the LCP film 2203. The RMLCM cell includessubstrates 2204, 2205 sandwiching the RMLCM layer 2206. Thisconfiguration has the advantage that the index matching oil bond canprovide a non-permanent bond, which allows for installation and removalof polarization cell for testing purposes. Adhesive can also be appliedat the edges (tacked) for a semi-permanent bond. In some embodiments theoil layer can be provided using a cell filled with oil.

FIG. 23 conceptually illustrates an example of a polarizationarchitecture 2300 based on a grating cell with the RMLCM gratingmaterial layer 2301 in direct contact with a bare LCP film 2302 inaccordance with an embodiment of the invention. The two films aresandwiched by the substrates 2303, 2304. This is a simple andcost-effective solution for implementing an LCP layer. Maintainingthickness control of the RMLCM layer using spacer beads can be difficultif the beads are embedded directly onto LCP layer. The embodiment ofFIG. 23 can required careful matching of the material properties of theRMLCM and LCP to avoid detrimental interactions between the RMLCM andthe LCP layers. In many embodiments, holographic exposure of the RMLCMlayer can be applied directly into the RMLCM and does not need to bethrough the LCP layer. If exposure construction through the LCP layer isunavoidable, pre-compensation of polarization rotation of the LCP layercan be made in some embodiments.

FIG. 24 conceptually illustrates a cross section view schematicallyshowing an example of polarization layer architecture 2400 in which abare LCP layer is bonded to a bare RMLCM layer in accordance with anembodiment of the invention. The apparatus includes an upper substrate2401, a bare LCP film 2402, adhesive layer 2403, an exposed RMLCM layer2404, and a lower substrate 2405. In many embodiments, the adhesivelayer can be Norland NOA65 adhesive or a similar adhesive.

FIG. 25 conceptually illustrates a cross section view schematicallyshowing an example of a polarization layer architecture 2500 using aRMLCM layer as a polarization layer in accordance with an embodiment ofthe invention. The apparatus includes an upper substrate 2501, an upperRMLCM layer 2502, a transparent spacer 2503, a lower RMLCM layer 2504,and a lower substrate 2505. One of the RMLCM layers can be used not onlyas the grating material, but also as a polarization rotation layer,using the inherent birefringent properties of RMLCM materials. The‘polarization rotation grating’ should have a period and/or k-vectordirection such that its diffraction is minimal. In some embodiments theRMLCM layer can be configure as a subwavelength grating. In someembodiments, the RMCM layer can be provided sandwiched between tworelease layers such that after curing the layer can be removed andre-applied elsewhere.

FIG. 26 conceptually illustrates an example of a polarization layerarchitecture 2600 that includes a feature for compensating forpolarization rotation introduced by birefringent gratings in accordancewith an embodiment of the invention. The apparatus includes an uppersubstrate 2601, a polarization control layer 2602, a transparentsubstrate 2603, a grating layer 2604, and a lower substrate 2605. Thegrating layer contains a first grating 2606A and a second grating 2606Bseparated by a clear region 2607. In some embodiments, the clear regioncan a polymer with refractive index similar to that of the substrates.In many embodiments other low refractive index materials may be used toprovide the clear region. The polarization control layer includesquarter wave retarding regions 2608A, 2608B and a polarizationcompensation region, which balances the polarization rotation introducedby the birefringent grating 2606A (in the case where the guide lightpropagates from grating 2606A to grating 2606B).

FIG. 27 conceptually illustrates a plan view schematically showing awaveguide display 2700 incorporating the features of the embodiment ofFIG. 26 in accordance with an embodiment of the invention. The waveguidedisplay 2700 includes a waveguide substrate 2701, an input grating 2702,a fold grating 2703, and an output grating 2704. Polarization controlregions 2705, 2706 apply compensation for grating depolarizationaccording to the principle of the embodiment of FIG. 26 .

FIG. 28 conceptually illustrates a cross section view schematicallyshowing an example of a polarization layer architecture 2800 containingan upper substrate 2801, an LCP layer 2802 with hard encapsulation layer2803, a RMLCM layer 2804, and a lower substrate 2805 in accordance withan embodiment of the invention. In many embodiments, the hardencapsulation layer or film can be designed to protect the delicate LCPfilm from mechanical contact, such that standard cleaning procedureswill not destroy the film. Advantageously, the hard encapsulation layercan employ a material resistant to spacer beads being pushed into itthrough the lamination process, as well as being chemically resistant toindex matching oil and adhesives.

FIG. 29 conceptually illustrates a cross section view schematicallyshowing an example of a polarization layer architecture 2900 containingan upper substrate 2901, an LCP layer 2902 with soft encapsulation layer2903, a RMLCM layer 2904, and a lower substrate 2905 in accordance withan embodiment of the invention. The polarization alignment film can beencapsulated with a soft encapsulation layer or film designed to protectthe delicate LCP film from mechanical contact, such that standardcleaning procedures such as drag wiping with iso-propyl alcohol, forexample, will not destroy the film. In some embodiments, the softencapsulation can provide some resistance to spacer beads during thelamination process.

FIG. 30 conceptually illustrates a plan view schematically showing afirst example 3000 of a two-region polymer film in accordance with anembodiment of the invention. This example using a non-encapsulated LCPfilm 3001 supported by a 0.5 mm thickness Eagle XG substrate ofdimensions 77.2 mm×47.2 mm. Region 1 is characterized by a fast axis 75°from horizontal and by quarter-wave retardance at 55° in-glass angle,45° ellipticity ±5°, for wavelength 524 nm. Region 2 is characterized bya fast axis 105° from horizontal and a quarter-wave retardance at 55°in-glass angle, 45° ellipticity ±5°, for wavelength 524 nm. Typically,region 1 and region 2 extend to the halfway point horizontally, ±2 mm.

FIG. 31 conceptually illustrates a plan view schematically showing asecond example 3100 of a two-region polymer film in accordance with anembodiment of the invention. This example uses encapsulation of the LCPlayer 3101 by a protective film 3102, said layers supported by a 0.5 mmthickness Eagle XG substrate of dimensions 77.2 mm×47.2 mm. Region 1 ischaracterized by a fast axis 75° from horizontal and by quarter-waveretardance at 55° in-glass angle, 45° ellipticity ±5°, for wavelength524 nm. Region 2 is characterized by a fast axis 105° from horizontaland by quarter-wave retardance at 55° in-glass angle, 45° ellipticity±5°, for wavelength 524 nm. Typically, Region 1 and region 2 extend tothe halfway point horizontally, ±2 mm. The encapsulation layer can sealthe polarization layer such that performance is unaffected when coveredby layer of oil such as Cargille Series A with refractive index 1.516.The encapsulation layer can seal the polarization layer such thatperformance is unaffected when covered by an additional layer of liquidcrystal-based photopolymer.

FIG. 32 conceptually illustrates a plan view schematically showing athird example 3200 of a two-region polymer film in accordance with anembodiment of the invention. This example uses glass encapsulation ofthe LCP. A 0.5 mm thickness Eagle XG substrate of dimensions 77.2mm×47.2 mm supports a LCP layer 3201, an adhesive layer 3202, and 0.2 mmthickness Willow glass cover 3203. Region 1 is characterized by a fastaxis 75° from horizontal and by quarter-wave retardance at 55° in-glassangle, 45° ellipticity ±5°, for wavelength 524 nm. Region 2 ischaracterized by a fast axis 105° from horizontal and by a quarter-waveretardance at 55° in-glass angle, 45° ellipticity ±5°, for wavelength524 nm. Advantageously, the glass for encapsulations of the LCP is 0.5mm EagleXG or 0.2 mm Willow glass. Typically, Region 1 and region 2extend to the halfway point horizontally, ±2 mm.

FIG. 33 conceptually illustrates a drawing showing the clear aperturelayout 3300 for the embodiments illustrated in FIGS. 30-32 in accordancewith an embodiment of the invention. The clear aperture is highlightedin the dashed line. All dimensions are in mm.

FIG. 34 conceptually illustrates a plan view 3400 schematically showingthe waveguide 3401 containing input 3402, fold 3403, and output 3404gratings based on the embodiments of FIGS.30-33, including the K-vectorsand alignment layer fast axis directions for each grating in accordancewith an embodiment of the invention. As shown in FIG. 34 , the K-vectorand fast axis directions are for the input grating K-vector: 30 degrees;for the fold grating K-vector: 270 degrees; and for the output gratingK-vector: 150 degrees.

The above description covers only some of the possible embodiments inwhich an LCP layer (or equivalent retarding layer) can be combined withan RMLCM layer in a waveguide structure. In many of the above describedembodiments, the substrates can be fabricated from 0.5 mm thicknessCorning Eagle XG glass. In some embodiments, thinner or thickersubstrates can be used. In several embodiments, the substrates can befabricated from plastic. In a number of embodiments, the substrates andoptical layers encapsulated by the said substrates can be curved. Any ofthe embodiments can incorporated additional layers for protection fromchemical contamination or damage incurred during processing andhandling. In some embodiments, additional substrate layers may beprovided to achieve a required waveguide thickness. In some embodiments,additional layers may be provided to perform at least one of thefunctions of illumination homogenization spectral filtering, angleselective filtering, stray light control, and debanding. In manyembodiments, the bare LCP layer can be bonded directly to a bare exposedRMLCM layer. In several embodiments, an intermediate substrate can bedisposed between the LCP layer and the RMLCM layer. In a number ofembodiments, the LCP layer can be combined with an unexposed layer ofRMLCM material. In many embodiments, layers of LCP, with or withoutencapsulation, can have haze characteristics <0.25%, and preferably 0.1%or less. It should be noted that the quoted haze characteristics arebased on bulk material scatter and are independent of surface scatterlosses, which are largely lost upon immersion. The LCP and encapsulationlayers can survive 100 C exposure (>80 C for thermal UM exposures). Inmany embodiments, the LCP encapsulation layer can be drag wipe resistantto permit layer cleaning. In the embodiments described above, there canbe constant retardance and no bubbles or voids within the film clearaperture. The LCP and adhesive layers can match the optical flatnesscriteria met by the waveguide substrates.

A color waveguide according to the principles of the invention wouldtypically include a stack of monochrome waveguides. The design may usered, green, and blue waveguide layers or, alternatively, red andblue/green layers. In some embodiments, the gratings are all passive,that is non-switching. In some embodiments, at least one of the gratingsis switching. In some embodiments, the input gratings in each layer areswitchable to avoid color crosstalk between the waveguide layers. Insome embodiments color crosstalk is avoided by disposing dichroicfilters between the input grating regions of the red and blue and theblue and green waveguides. In some embodiments, the thickness of thebirefringence control layer is optimized for the wavelengths of lightpropagating within the waveguide to provide the uniform birefringencecompensation across the spectral bandwidth of the waveguide display.Wavelengths and spectral bandwidths bands for red, green, bluewavelengths typically used in waveguide displays are red: 626 nm±9 nm,green: 522 nm±18 nm and blue: 452 nm±11 nm. In some embodiments, thethickness of the birefringence control layer is optimized fortrichromatic light.

In many embodiments, the birefringence control layer is provided by asubwavelength grating recorded in HPDLC. Such gratings are known toexhibit the phenomenon of form birefringence and can be configured toprovide a range of polarization functions including quarter wave andhalf wave retardation. In some embodiments, the birefringence controllayer is provided by a liquid crystal medium in which the LC directorsare aligned by illuminating an azo-dye doped alignment layer withpolarized or unpolarized light. In a number of embodiments, abirefringence control layer is patterned to provide LC directororientation patterns with submicron resolution steps. In sameembodiments, the birefringence control layer is processed to providecontinuous variation of the LC director orientations. In severalembodiments, a birefringence control layer provided by combining one ormore of the techniques described above is combined with a rubbingprocess or a polyimide alignment layer. In some embodiments, thebirefringence control layer provides optical power. In a number ofembodiments, the birefringence control layer provides a gradient indexstructure. In several embodiments, the birefringence control layer isprovided by a stack containing at least one HPDLC grating and at leastone alignment layer. In many embodiments, the birefringent grating mayhave rolled k-vectors. The K-vector is a vector aligned normal to thegrating planes (or fringes) which determines the optical efficiency fora given range of input and diffracted angles. Rolling the K-vectorsallows the angular bandwidth of the grating to be expanded without theneed to increase the waveguide thickness. In many embodiments, thebirefringent grating is a fold grating for providing exit pupilexpansion. The fold grating may be based on any of the embodimentsdisclosed in PCT Application No.: PCT/GB2016000181 entitled WAVEGUIDEDISPLAY and embodiments discussed in the other references give above.

In some embodiments, the apparatus is used in a waveguide design toovercome the problem of laser banding. A waveguide according to theprinciples of the invention can provide a pupil shifting means forconfiguring the light coupled into the waveguide such that the inputgrating has an effective input aperture which is a function of the TIRangle. Several embodiments of the pupil shifting means will bedescribed. The effect of the pupil shifting means is that successivelight extractions from the waveguide by the output grating integrate toprovide a substantially flat illumination profile for any lightincidence angle at the input grating. The pupil shifting means can beimplemented using the birefringence control layers to vary at least oneof amplitude, polarization, phase, and wavefront displacement in 3Dspace as a function of incidence light angle. In each case, the effectis to provide an effective aperture that gives uniform extraction acrossthe output grating for any light incidence angle at the input grating.In some embodiments, the pupil shifting means is provided at least inpart by designing the optics of the input image generator to have anumerical aperture (NA) variation ranging from high NA on one side ofthe microdisplay panel varying smoothly to a low NA at the other sideaccording to various embodiments, such as those similar to onesdisclosed in PCT Application No.: PCT/GB2016000181 entitled WAVEGUIDEDISPLAY, the disclosure of which is hereby incorporated in its entirety.Typically, the microdisplay is a reflective device.

In some embodiments, the grating layer may be broken up into separatelayers. The number of layers may then be laminated together into asingle waveguide substrate. In many embodiments, the grating layercontains several pieces, including the input coupler, the fold grating,and the output grating (or portions thereof) that are laminated togetherto form a single substrate waveguide. The pieces may be separated byoptical glue or other transparent material of refractive index matchingthat of the pieces. In several embodiments, the grating layer may beformed via a cell making process by creating cells of the desiredgrating thickness and vacuum filling each cell with SBG material foreach of the input coupler, the fold grating and the output grating. Inone embodiment, the cell is formed by positioning multiple plates ofglass with gaps between the plates of glass that define the desiredgrating thickness for the input coupler, the fold grating and the outputgrating. In one embodiment, one cell may be made with multiple aperturessuch that the separate apertures are filled with different pockets ofSBG material. Any intervening spaces may then be separated by aseparating material (e.g., glue, oil, etc.) to define separate areas. Inone embodiment, the SBG material may be spin-coated onto a substrate andthen covered by a second substrate after curing of the material. Byusing a fold grating, the waveguide display advantageously requiresfewer layers than previous systems and methods of displaying informationaccording to some embodiments. In addition, by using a fold grating,light can travel by total internal refection within the waveguide in asingle rectangular prism defined by the waveguide outer surfaces whileachieving dual pupil expansion. In another embodiment, the inputcoupler, the gratings can be created by interfering two waves of lightat an angle within the substrate to create a holographic wave front,thereby creating light and dark fringes that are set in the waveguidesubstrate at a desired angle. In some embodiments, the grating in agiven layer is recorded in stepwise fashion by scanning or stepping therecording laser beams across the grating area. In some embodiments, thegratings are recorded using mastering and contact copying processcurrently used in the holographic printing industry.

In many embodiments, the gratings are Bragg gratings recorded inholographic polymer dispersed liquid crystal (HPDLC) as alreadydiscussed, although SBGs may also be recorded in other materials. In oneembodiment, SBGs are recorded in a uniform modulation material, such asPOLICRYPS or POLIPHEM having a matrix of solid liquid crystals dispersedin a liquid polymer. The SBGs can be switching or non-switching innature. In its non-switching form a SBG has the advantage overconventional holographic photopolymer materials of being capable ofproviding high refractive index modulation due to its liquid crystalcomponent. Exemplary uniform modulation liquid crystal-polymer materialsystems are disclosed in United State Patent Application PublicationNo.: US2007/0019152 by Caputo et al and PCT Application No.:PCT/EP2005/006950 by Stumpe et al., both of which are incorporatedherein by reference in their entireties. Uniform modulation gratings arecharacterized by high refractive index modulation (and hence highdiffraction efficiency) and low scatter. In some embodiments at leastone of the gratings is a surface relief grating. In some embodiments atleast one of the gratings is a thin (or Raman-Nath) hologram,

In some embodiments, the gratings are recorded in a reverse mode HPDLCmaterial. Reverse mode HPDLC differs from conventional HPDLC in that thegrating is passive when no electric field is applied and becomesdiffractive in the presence of an electric field. The reverse mode HPDLCmay be based on any of the recipes and processes disclosed in PCTApplication No.: PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHICPOLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES. The grating maybe recorded in any of the above material systems but used in a passive(non-switching) mode. The fabrication process can be identical to thatused for switched but with the electrode coating stage being omitted. LCpolymer material systems may be used for their high index modulation. Insome embodiments, the gratings are recorded in HPDLC but are notswitched.

In many embodiments, a waveguide display according to the principles ofthe invention may be integrated within a window, for example, awindscreen-integrated HUD for road vehicle applications. In someembodiments, a window-integrated display may be based on the embodimentsand teachings disclosed in U.S. Provisional Patent Application No.62/125,064 entitled OPTICAL WAVEGUIDE DISPLAYS FOR INTEGRATION INWINDOWS and U.S. patent application Ser. No. 15/543,016 entitledENVIRONMENTALLY ISOLATED WAVEGUIDE DISPLAY. In some embodiments, awaveguide display according to the principles of the invention mayincorporate a light pipe for providing beam expansion in one directionbased on the embodiments disclosed in U.S. patent application Ser. No.15/558,409 entitled WAVEGUIDE DEVICE INCORPORATING A LIGHT PIPE. In someembodiments, the input image generator may be based on a laser scanneras disclosed in U.S. Pat. No. 9,075,184 entitled COMPACT EDGEILLUMINATED DIFFRACTIVE DISPLAY. The embodiments of the invention may beused in wide range of displays including HMDs for AR and VR, helmetmounted displays, projection displays, heads up displays (HUDs), HeadsDown Displays, (HDDs), autostereoscopic displays and other 3D displays.Some of the embodiments and teachings of this disclosure may be appliedin waveguide sensors such as, for example, eye trackers, fingerprintscanners and LIDAR systems and in illuminators and backlights.

It should be emphasized that the drawings are exemplary and that thedimensions have been exaggerated. For example, thicknesses of the SBGlayers have been greatly exaggerated. Optical devices based on any ofthe above-described embodiments may be implemented using plasticsubstrates using the materials and processes disclosed in PCTApplication No.: PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHICPOLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES. In someembodiments, the dual expansion waveguide display may be curved.

Although the description has provided specific embodiments of theinvention, additional information concerning the technology may be foundin the following patent applications, which are incorporated byreference herein in their entireties: U.S. Pat. No. 9,075,184 entitledCOMPACT EDGE ILLUMINATED DIFFRACTIVE DISPLAY, U.S. Pat. No. 8,233,204entitled OPTICAL DISPLAYS, PCT Application No.: US2006/043938, entitledMETHOD AND APPARATUS FOR PROVIDING A TRANSPARENT DISPLAY, PCTApplication No.: GB2012/000677 entitled WEARABLE DATA DISPLAY, U.S.patent application Ser. No. 13/317,468 entitled COMPACT EDGE ILLUMINATEDEYEGLASS DISPLAY, U.S. patent application Ser. No. 13/869,866 entitledHOLOGRAPHIC WIDE ANGLE DISPLAY, and U.S. patent application Ser. No.13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY, U.S. patentapplication Ser. No. 14/620,969 entitled WAVEGUIDE GRATING DEVICE, U.S.patent application Ser. No. 15/553,120 entitled ELECTRICALLY FOCUSTUNABLE LENS, U.S. patent application Ser. No. 15/558,409 entitledWAVEGUIDE DEVICE INCORPORATING A LIGHT PIPE, U.S. patent applicationSer. No. 15/512,500 entitled METHOD AND APPARATUS FOR GENERATING INPUTIMAGES FOR HOLOGRAPHIC WAVEGUIDE DISPLAYS, U.S. Provisional PatentApplication No. 62/123,282 entitled NEAR EYE DISPLAY USING GRADIENTINDEX OPTICS, U.S. Provisional Patent Application No. 62/124,550entitled WAVEGUIDE DISPLAY USING GRADIENT INDEX OPTICS, U.S. ProvisionalPatent Application No. 62/125,064 entitled OPTICAL WAVEGUIDE DISPLAYSFOR INTEGRATION IN WINDOWS, U.S. patent application Ser. No. 15/543,016entitled ENVIRONMENTALLY ISOLATED WAVEGUIDE DISPLAY, U.S. ProvisionalPatent Application No. 62/125,089 entitled HOLOGRAPHIC WAVEGUIDE LIGHTFIELD DISPLAYS, U.S. Pat. No. 8,224,133 entitled LASER ILLUMINATIONDEVICE, U.S. Pat. No. 8,565,560 entitled LASER ILLUMINATION DEVICE, U.S.Pat. No. 6,115,152 entitled HOLOGRAPHIC ILLUMINATION SYSTEM, PCTApplication No.: PCT/GB2013/000005 entitled CONTACT IMAGE SENSOR USINGSWITCHABLE BRAGG GRATINGS, PCT Application No.: PCT/GB2012/000680,entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTALMATERIALS AND DEVICES, PCT Application No.: PCT/GB2014/000197 entitledHOLOGRAPHIC WAVEGUIDE EYE TRACKER, PCT/GB2013/000210 entitled APPARATUSFOR EYE TRACKING, PCT Application No.:GB2013/000210 entitled APPARATUSFOR EYE TRACKING, PCT/GB2015/000274 entitled HOLOGRAPHIC WAVEGUIDEOPTICALTRACKER, U.S. Pat. No. 8,903,207 entitled SYSTEM AND METHOD OFEXTENDING VERTICAL FIELD OF VIEW IN HEAD UP DISPLAY USING A WAVEGUIDECOMBINER, U.S. Pat. No. 8,639,072 entitled COMPACT WEARABLE DISPLAY,U.S. Pat. No. 8,885,112 entitled COMPACT HOLOGRAPHIC EDGE ILLUMINATEDEYEGLASS DISPLAY, U.S. patent application Ser. No. 16/086,578 entitledMETHOD AND APPARATUS FOR PROVIDING A POLARIZATION SELECTIVE HOLOGRAPHICWAVEGUIDE DEVICE, U.S. Provisional Patent Application No. 62/493,578entitled WAVEGUIDE DISPLAY APPARATUS, PCT Application No.:PCT/GB2016000181 entitled WAVEGUIDE DISPLAY, U.S. Patent Application No.62/497,781 entitled APPARATUS FOR HOMOGENIZING THE OUTPUT FROM AWAVEGUIDE DEVICE, U.S. Patent Application No. 62/499,423 entitledWAVEGUIDE DEVICE WITH UNIFORM OUTPUT ILLUMINATION.

DOCTRINE OF EQUIVALENTS

The construction and arrangement of the systems and methods as shown inthe various exemplary embodiments are illustrative only. Although only afew embodiments have been described in detail in this disclosure, manymodifications are possible (for example, variations in sizes,dimensions, structures, shapes and proportions of the various elements,values of parameters, mounting arrangements, use of materials, colors,orientations, etc.). For example, the position of elements may bereversed or otherwise varied, and the nature or number of discreteelements or positions may be altered or varied. Accordingly, all suchmodifications are intended to be included within the scope of thepresent disclosure. The order or sequence of any process or method stepsmay be varied or re-sequenced according to alternative embodiments.Other substitutions, modifications, changes, and omissions may be madein the design, operating conditions and arrangement of the exemplaryembodiments without departing from the scope of the present disclosure.

What is claimed is:
 1. A waveguide comprising: at least one waveguidesubstrate; at least one grating; at least one polarization modifyinglayer, wherein the at least one polarization modifying layer is a liquidcrystal and polymer system aligned using directional ultravioletradiation; a light source for outputting light; an input coupler fordirecting the light into total internal reflection paths within thewaveguide; and an output coupler for extracting light from thewaveguide, wherein the interaction of the light with the at least onepolarization modifying layer and the at least one grating provides apredefined characteristic of light extracted from the waveguide.
 2. Awavequide comprising: at least one wavequide substrate; at least onegrating; at least one polarization modifying layer; a light source foroutputting light; an input coupler for directing the light into totalinternal reflection paths within the wavequide; and an output couplerfor extracting light from the waveguide, wherein the interaction of thelight with the at least one polarization modifying layer and the atleast one grating provides a predefined characteristic of lightextracted from the wavequide, wherein the at least one grating includesa birefringent grating formed in a liquid crystal and polymer system,and wherein the at least one polarization modifying layer influences thealignment of LC directors in the birefringent grating.
 3. A wavequidecomprising: at least one waveguide substrate; at least one grating; atleast one polarization modifying layer, wherein the at least onepolarization modifying layer comprises at least one stack of refractiveindex layers disposed on at least one optical surface of the waveguide,and wherein at least one layer in the stack of refractive index layershas an isotropic refractive index and at least one layer in the stack ofrefractive index layers has an anisotropic refractive index; a lightsource for outputting light; an input coupler for directing the lightinto total internal reflection paths within the waveguide; and an outputcoupler for extracting light from the waveguide, wherein the interactionof the light with the at least one polarization modifying layer and theat least one grating provides a predefined characteristic of lightextracted from the waveguide.
 4. A waveguide comprising: at least onewaveguide substrate; at least one grating; at least one polarizationmodifying layer, wherein the at least one polarization modifying layerprovides optical power; a light source for outputting light; an inputcoupler for directing the light into total internal reflection pathswithin the waveguide; and an output coupler for extracting light fromthe waveguide, wherein the interaction of the light with the at leastone polarization modifying layer and the at least one grating provides apredefined characteristic of light extracted from the waveguide.
 5. Thewaveguide of claim 1, wherein the predefined characteristic comprises atleast one of: uniform illumination and uniform polarization over theangular range of the light.
 6. The waveguide of claim 1, wherein the atleast one polarization modifying layer provides compensation forpolarization rotation introduced by the at least one grating along atleast one direction of light propagation within the waveguide.
 7. Thewaveguide of claim 1, wherein the at least one polarization modifyinglayer is a liquid crystal and polymer material system.
 8. The waveguideof claim 1, wherein the interaction of light with the at least onepolarization modifying layer provides at least one of: an angular orspectral bandwidth variation; a polarization rotation; a birefringencevariation; an angular or spectral dependence of at least one of beamtransmission or polarization rotation; or a light transmission variationin at least one direction in the plane of the waveguide substrate. 9.The waveguide of claim 1, wherein the at least one polarizationmodifying layer is aligned by at least one of: electromagneticradiation; electrical or magnetic fields; mechanical forces; chemicalreaction; or thermal exposure.
 10. The waveguide of claim 1, wherein thepredefined characteristic varies across the waveguide.
 11. The waveguideof claim 1, wherein the at least one polarization modifying layer has ananisotropic refractive index.
 12. The waveguide of claim 1, wherein theat least one polarization modifying layer is formed on at least oneinternal or external optical surface of the waveguide.
 13. The waveguideof claim 1, wherein the predefined characteristic results from thecumulative effect of the interaction of the light with the at least onepolarization modifying layer and the at least one grating along at leastone direction of light propagation within the waveguide.
 14. Thewaveguide of claim 1, wherein the at least one polarization modifyinglayer is reflective.
 15. The waveguide of claim 1, wherein the at leastone grating comprises two or more gratings configured as a stack. 16.The waveguide of claim 1, wherein the at least one polarizationmodifying layer provides an environmental isolation layer for thewaveguide.
 17. The waveguide of claim 1, wherein the at least onepolarization modifying layer has a gradient index structure.
 18. Thewaveguide of claim 1, wherein the at least one polarization modifyinglayer is formed by stretching a layer of an optical material tospatially vary its refractive index in the plane of the waveguidesubstrate.
 19. The waveguide of claim 1, wherein the light sourceprovides collimated light in angular space.
 20. The waveguide of claim1, wherein at least one of the input coupler and output couplercomprises a birefringent grating.
 21. The waveguide of claim 1, whereinthe at least one grating is formed in a birefringent material.
 22. Thewaveguide of claim 1, wherein the at least one grating is a surfacerelief grating.
 23. The waveguide of claim 1, wherein the at least onegrating is a fold grating.
 24. The waveguide of claim 1, wherein the atleast one grating comprises two or more gratings multiplexed in a layer.